Inflow and infiltration analysis, along with evaluation of the sanitary sewer collection system helped the City of Largo significantly decrease wet weather flows, minimize the potential for sanitary sewer overflows (SSOs), increase collection system reliability, and diminish public health risks and environmental pollution.
The evaluations came about following Hurricane Hermine. The City of Largo received more than a foot of rain in just 24 hours. Two days after the storm passed through, some manholes were still overflowing.
Even before Hurricane Hermine, Largo was plagued with recurring SSOs during periods of heavy rainfall. The City determined that the primary cause of the recurring SSOs was related to pump station and force main capacities and quickly undertook a $38-million program to upgrade that part of their system.
After the enhancements were made there were still six lift station service areas that experienced recurring SSOs. Although they were considered minimal releases, the City wanted to alleviate the SSO problem altogether, thereby ensuring customer service, decrease risk to public health and environmental quality. The City needed to implement an inflow and infiltration (I&I) reduction program in the six lift station service areas that would minimize SSOs and accommodate future capacity needs. In addition, the necessary improvements had to be completed before previously allocated funding expired.
To accomplish this, Largo embarked on a new approach to SSO abatement that was founded entirely on the projected effects of I&I source remediation, using hydraulic capacity analysis as the primary driver. The new program streamlined the manner in which necessary data sets are collected, analyzed and developed. The approach included completion of comprehensive sewer system evaluation survey (SSES) activities and hydraulic capacity analysis. The SSES tasks identified both stormwater-related inflow and excessive groundwater infiltration sources through smoke/dye testing and manhole inspections/night flow isolations and CCTV inspections. The hydraulic modeling initiative was completed using flow/rainfall/groundwater data captured over an 8-month period.
Upon completion of the SSES activities, a data assessment was conducted. The results were merged with the outcome of the modeling effort. Pipes that were determined to be at capacity, based on wet weather flow, were independently evaluated to determine what flow reduction was necessary to eliminate the potential for an SSO. If it was determined that the necessary wet weather flow reduction could be achieved, then the required collection system improvements were identified and the costs to complete them were projected. Once the capacity achievement analysis was completed, a prioritized infrastructure renewal and replacement program was created and the improvements completed utilizing in-place “on-call” construction contracts.
In summary, the City’s SSO abatement program evolved into a hybrid “find and fix” initiative that combined true partnering between the City, engineering consultant and multiple contractors to meet an aggressive schedule within a limited budget.